The lifetimes of SF-6 ions produced in K(np)/SF6 collisions at high n are being investigated as a function of target temperature over the range 300K to 600K. At room temperature collisions are found to lead predominantly to the formation of long-lived SF-6 ions with lifetimes tau ≳ 1ms. As the target temperature is raised long-lived (tau ≳ 0.5ms) ions are still observed but their mean lifetime is reduced. In addition, the growth of a short-lived ion signal (tau ≳ 10 mus) is evident which, by 600K, accounts for ∼45% of the total SF-6 ion signal. These lifetimes are compared with those obtained using quasi-equilibrium theory and calculated SF-6 vibrational frequencies.

Photoelectron spectroscopy of negatively, charged clusters of aluminum, copper, silver, and gold reveals electronic shell structure and electron energy band development. Photodetachment experiments on Au$\sb6\sp-$ also suggests that an image-bound state of the anion exists at the detachment threshold. Al$\sb{\rm x}$, Cu$\sb{\rm x}$, Ag$\sb{\rm x}$, and Au$\sb{\rm x}$ all show closed shells in their electron affinities for clusters with x $<$ 21. The electron affinities of Au$\sb{\rm x}$ for 20 $<$ x $<$ 60 have three spherical shells at x = 20, 34, and 58, all of which are dramatically manifested in dips of 1.2, 0.65, and 0.75 eV when compared to their adjacent lower mass neighbors. The photoelectron spectra of Ag$\sb{\rm x}\sp-$, 30 $<$ x $<$ 58, have cleanly resolved peaks that are a result of level structure in the density-of-states and they are predicted by the Nilsson-Clemenger formalism of shell theory. Copper clusters in the same size range show similar behavior although not as pronounced. The onset of the 3d-band in copper is monotonically increasing with cluster size and at Cu$\sb{410}$ the band is only 0.6 eV from its bulk value. The monotonic increase is a result of the expanding sphere of charge that induces a dipole as the electron leaves the vicinity of the cluster. Sharp features in the 3d-band onset are very similar to bulk photoemission spectra and suggest that these structures are beginning to show crystalline character. The induced dipole also causes at least one weakly bound state to exist for Au$\sb6\sp-$ at the detachment threshold. Photodetachment spectroscopy shows a 30 cm$\sp{-1}$ phase shift between the 1- and 2-photon ejected electrons. The phase shift may be due to a sequence congestion of vibrational states between the ground and image-bound electronic states of Au$\sb6\sp-$.

The properties of fluids near a fluid-solid interface are important in many processes, such as: wettability as related to oil recovery and environmental cleanup, biochemical separation, bio-compatibility of materials, membrane separation, adsorption in porous solids and micro- or nanomanufacturing of thin films. However, little of the past simulation and theoretical work in the field has considered associating fluids. This work is a Metropolis Monte Carlo study of a simple model of water against a hard plate. The potential model is a spherical hard core with four highly anisotropic square well sites placed in tetrahedral symmetry. Reported are density of spheres and fraction of unbonded spheres as a function of distance from the plate. Computed wall densities are compared against those calculated by the wall density law. Also, the beginning of an outline of a suitable density functional theory for this fluid is presented.

A study of concentrated aqueous alkali halide solutions is made at the molecular level, through modeling and computer simulation of their structural and thermodynamic properties. It is found that the HNC approximation is the best integral equation theory to predict such properties within the framework of the primitive model (PM). The intrinsic limitations of the PM in describing ionic association and hydration effects are addressed and discussed in order to emphasize the need for explicitly including the water molecules in the treatment of aqueous electrolyte solutions by means of a civilized model (CM). As a step toward developing a CM as simple as possible, it is shown that a modified version of the SPC model of liquid water in which the Lennard-Jones interaction between intermolecular oxygen sites is replaced by a hard core interaction, is still successful enough to predict the degree of hydrogen bonding of real water. A simple civilized model (SCM) (in which the ions are treated as hard spheres interacting through Coulombic potentials and the water molecules are simulated using the simplified SPC model) is introduced in order to study the changes in the structural features of various aqueous alkali halide solutions upon varying both the concentration and the size of the ions. Both cations and anions are found to be solvated by the water molecules at expense of a breakdown in the hydrogen-bonded water network. Hydration numbers are reported for the first time for NaBr and KBr, and the first simulation-based estimates for LiBr, NaI and KI are also obtained. In several cases, values of the hydration numbers based on the SCM are found to be in excellent agreement with available experimental results obtained from x-ray diffraction measurements. Finally, it is shown that a neoprimitive model (NPM) can be developed by incorporating some of the structural features seen in the SCM into the short-range part of the PM interionic potential via a shielded square well whose width and depth's temperature coefficient can be determined from a fit of experimental data for the osmotic coefficient and heat of dilution, respectively.

The temperature dependence of conversion of $\rm He(2\sp3S\sb1)$ metastable atoms to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+})$ metastable molecules in the three-body reaction $\rm He(2\sp3S\sb1)+ 2He(1\sp1S\sb0)\to He\sb2(a\sp3\Sigma\sbsp{u}{+}) + He(1\sp1S\sb0)$ has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the $\rm He(2\sp3S\sb1)-He(1\sp1S\sb0)$ interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient k$\sb{\rm s}($T) is accurately described by $\rm k\sb{s}(T) = \lbrack 87.4\ T\ exp({-750}/T) + 4.1 T\ exp({-200}/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}$. The first activation energy, 750 $\pm$ 70K (63 $\pm$ 6meV), is equal to the known $\rm He(2\sp3S\sb1)$-$\rm He(1\sp1S\sb0)$ repulsive barrier height. The second activation energy is 17 $\pm$ 2 meV. The temperature dependences of the rate constants for collision-induced mixing among $\rm He(2\sp3P\sb{J,m\sb{J}})$ levels, and for conversion of He(2$\sp3$P) atoms to $\rm He\sb2(b\sp3\Pi\sb{g})$ molecules in the three body reaction $\rm He(2\sp3P) + 2He(1\sp1S)\to He\sb2(b\sp3\Pi\sb{g}) + He(1\sp1S)$ have been investigated over the range 1.4$\sim$300K. The measured thermally-averaged cross section for $\rm He(2\sp3P\sb{J,m\sb{J}})$ mixing in collisions with ground state helium atoms are described by the function $\rm\sigma\sb{pm}(T) = (4.4 + 20.6/T\sp{1/3})\times 10\sp{-15}cm\sp2,$ and can be understood in terms of Langevin theory. The measured rate coefficients for the three body reaction exhibit a strong inverse temperature dependence, $\rm k\sb{p}(T) = (0.04 + 2.18/T)\times 10\sp{-30}\ cm\sp6{\cdot}s\sp{-1},$ which suggests that, unlike conversion of $\rm He(2\sp3S\sb1)$ to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+}),$ there is no activation energy required for this reaction. A magneto-optical trap for helium 2$\sp3$S metastable atoms has been designed and constructed, utilizing superconducting magnet gradient coils and a Ti:Sapphire ring laser for pumping the helium 2$\sp3$S-2$\sp3$P transition. He(2$\sp3$S) atoms are produced by a weak discharge in helium gas at temperature 1.3K. The discharge products flow through an orifice into the trap cell, where the He(2$\sp3$S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experimental results suggest that $\sim$10$\sp6$ atoms are trapped, with a trap lifetime of about 0.2 sec limited by He(2$\sp3$S) - He(2$\sp3$P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be trapped and cooled for much longer times in a near-perfect vacuum. Measurements of decay times of the trapped atoms should yield rates for $\rm \sp{4,3}He(2\sp3S)$ - $\sp{4,3}$He(2$\sp3$S) and resonantly-enhanced He(2$\sp3$S) - He(2$\sp3$P) Penning reactions in the ultra-cold quantum regime, and perhaps the He(2$\sp3$S) natural lifetime.

A method is described which utilizes the difference in vapor pressure between C$\sb{60}$ and heavier fullerenes to produce C$\sb{60}$ powder with a purity of 99.97%. Using the material from this process allows for the growth of high purity polycrystalline C$\sb{60}$ thin films. These films are characterized using Raman spectroscopy and temperature dependent photoluminescence. The temperature dependence of the photoluminescence spectra indicates that both intermolecular and intramolecular processes are involved in the radiative recombination of the excited states. A model is proposed to describe the temperature dependence of the photoluminescence. A possible interpretation of this model is that there is a barrier to the formation of self trapped excitons.

The spectra of the high-lying vibrational levels of the $A\sp1\Sigma\sbsp{u}{+}$ and $1\sp3\Sigma\sbsp{g}{+}$ singly excited states of both $\sp6\rm Li\sb2$ and $\sp7\rm Li\sb2$ are obtained via photoassociation of colliding ultracold atoms confined in a magneto-optical trap. The least bound state of the $a\sp3\Sigma\sbsp{u}{+}$ ground state potential, obtained by two-photon photoassociative spectroscopy, is also presented for both $\sp7\rm Li\sb2$ and $\sp6\rm Li\sb2$. The vibrational levels have resolved hyperfine structure, whose relative energy splittings and transition strengths are accurately modeled. The photoassociative spectra are used to precisely determine both the singlet and triplet s-wave scattering lengths for both isotopes, which are important parameters in systems designed to observe quantum degenerate effects and Bose-Einstein condensation.

We have used the density functional ab initio method to study the reactivity of small clusters (ammonia reacting with GaAs clusters), and the effects of electric field on adatom-surface interactions (H and Al adsorbed on Si(111) surface). In the case of NH$\sb3$ reactivity at various sites of a Ga$\rm\sb5As\sb5$ cluster, a strong correlation is found between the adsorbate-cluster binding energy and charge transfer. Ionizing the bare cluster to a cationic charge state enables multiple chemisorption of ammonia, which is much less favored on a neutral cluster and can be prohibited on a negatively charged cluster. In the case H and Al adsorbed on a Si(111) surface, the influence of an external electric field, such as that present in a scanning tunnelling microscope (STM), on the chemisorption bond is investigated. The changes in charge distributions, vibrational frequencies and adsorbate desorption barriers are calculated as a function of the strength and direction of the electric field. We find that the characteristics of the chemisorption bond can, to a large extent, be controlled through the externally applied field.

Macroscopic synthetic techniques for producing fullerene molecules have been developed which include the carbon arc and laser vaporization approaches. The "pentagon rule" kinetic growth mechanism for the fullerenes is proposed to explain this facile process. The reactor technology for generating fullerene molecules is generalized. Resonant two-photon ionization measurements have yielded the triplet state lifetimes and electronic spectroscopy of C$\sb{60}$ and C$\sb{70}$ in a supersonic molecular beam. The technique and results are described in detail. An unusual type of C$\sb{60}$ $\cdot$ Ar$\sp{+}$ complex has been detected through fragmentation studies in a tandem time-of-flight mass spectrometer. Rotational stabilization is proposed as the mechanism leading to the long lifetime of this and similar species.

Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl$\sb4$ and CF$\sb3$I. The transient negative ions are formed during Rydberg electron transfer reactions of the type$${\rm K}(n{\rm p})+AB\to{\rm K}\sp{+}+AB\sp{-*}\to{\rm K}\sp{+}+A\sp{-}+B$$For low values of principal quantum number $n\ (n\ \sbsp{\sim}{<}\ 20)$ the electrostatic interaction between the ${\rm K}\sp{+}-AB\sp{-*}$ and ${\rm K}\sp{+}+A\sp{-}$ ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the ${\rm K}\sp{+}+A\sp{-}$ ions are dependent on the lifetime of the $AB\sp{-*}$ intermediate ion. The $AB\sp{-*}$ lifetime is determined by comparing product ion distributions, determined using position sensitive detection techniques, with the results of a Monte Carlo simulation that models the kinematics of the reaction. The data show that the lifetime of the excited $\rm CF\sb3\Gamma\sp{*}$ intermediate formed by Rydberg electron capture is short, $\sbsp{\sim}{<}2$ ps, and that the lifetime of the CCl$\sbsp{4}{-*}$ intermediate is $7.5\pm2.5$ ps.